1. Field of the Invention
The present invention relates to a transistor, in particular, a transistor in which a channel is formed in an oxide semiconductor layer. Further, the present invention relates to a semiconductor device including the transistor, for example, an active matrix display device including the transistor in each pixel. Note that in this specification, a semiconductor device refers to any device that operates by utilizing semiconductor characteristics.
2. Description of the Related Art
In recent years, a metal oxide having semiconductor characteristics, which is called an oxide semiconductor, has attracted attention as a material of a transistor. Metal oxides are used for a variety of applications. For example, indium oxide is used as a material of a pixel electrode in a liquid crystal display device. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide, and transistors in each of which a channel is formed using such a metal oxide layer having semiconductor characteristics have been known (Patent Documents 1 and 2).